In recent years, intravenously administered ultrasound contrast agents have been commercialized, and a contrast echo technique called contrast harmonic imaging (CHI) has been performed using ultrasound diagnostic apparatuses. Contrast echo techniques are intended for evaluation of hemodynamics by intravenously infusing ultrasound contrast agents to enhance blood-flow signals in cardiac and hepatic tests, for example. Many types of ultrasound contrast agents have microbubbles serving as a reflection source. However, due to its delicate nature as the base material, bubbles are broken by the mechanical action of ultrasound waves, resulting in reduction in the intensity of signals from the scanned surface, even when the ultrasound waves are irradiated at the level of ordinary diagnosis.
For the above reason, to observe the dynamic state of reflux flow in real time, it is necessary to relatively reduce the breakdown of the bubbles due to scanning by performing imaging with transmission of ultrasound waves with low acoustic pressure, for example. During such imaging with transmission of ultrasound waves with low acoustic pressure, the signal/noise (S/N) ratio is lowered. To compensate this, various signal processing methods have been developed such as phase modulation (PM), amplitude modulation (AM), and amplitude modulation/phase modulation (AMPM). The above-mentioned imaging methods enable real-time display of a contrasted image with a high S/N ratio. Ultrasound contrast imaging is used for detailed examination of microstructures (microvascular structures, for example) that cannot be visualized by X-ray computed tomography (CT) apparatuses or magnetic resonance imaging (MRI) apparatuses in view of real-time performance and high spatial resolution. Ultrasound contrast imaging may also be useful for differential diagnosis because it enables observation of irregular courses of tumor vessels and nutrient vessels. Ultrasound contrast imaging is used for superficial regions in addition to abdominal regions.
When observing a microlesion, an ultrasonic probe with high frequency (6 MHz or higher) is used to obtain spatial resolution although it reduces sensitivity in a deep region. When scanning a patient with a thick abdominal wall also, depth sensitivity is reduced. This is because the ultrasound waves transmitted with high frequency causes significant frequency-dependent attenuation, and thereby the observable region (penetration) is limited to a shallow area. To secure an observation depth, frequency is lowered generally although it lowers spatial resolution. Clinically, it is necessary to observe a lesion with the spatial resolution being maintained in the deep region. However, this may not be satisfied in some cases.